1. Field of the Invention
The invention relates to fabrication of glass optical fiber, including fabrication of preforms for forming such fiber.
2. Discussion of the Related Art
As the use of glass optical fiber has increased, the demand for stronger, more durable fibers with improved optical properties has similarly grown. Loss mechanisms and structural faults in optical fiber, e.g., bubbles, typically result from imperfections and impurities existing in the glass preform from which the fiber is drawn, and much effort has gone into finding useful ways to detect, monitor, and reduce, remove, or eliminate these imperfections and impurities. Problems encountered in the detection and monitoring of impurities include the time required to obtain an accurate measurement, and the difficulty with which that measurement is made. For example, hydrogen impurities in silica articles for preform manufacture, e.g., tubes or core rods, are typically measured by forming a preform from the article or articles, drawing fiber from the preform, and measuring the OH absorption of light transmitted through the fiber. Thus, it is not possible to determine whether the material contains undesired amount of hydrogen without going through the entire fiber fabrication process. Similarly, carbon content of silica materials is generally measured by placing a relatively large sample, e.g., tens to hundreds of grams, into a furnace, baking and decomposing the silica, and measuring the evolved carbon dioxide at high temperatures (greater than 2000.degree. C.). This method, however, takes a relatively long time, does not offer as high a precision as desired, demands a relatively large sample, and requires expensive apparatus.
Improved techniques for determining impurity content of glass preforms and/or the resultant fiber are therefore desired.